1. Field of the Invention
The present invention relates to a combustion control apparatus for a burner used in a boiler or other combustion facilities, which is arranged to detect a combustion condition of a flame generated from the burner on the basis of the ionic current or light of the flame, process the detected signal in an electrical circuit and then control the fuel and air flow rates to preset values, thereby maintaining the combustion in the best condition.
2. Prior Art
It is preferable that burners for burning liquid or gas fuel be maintained in an optimal combustion condition. Prior arts contrived to maintain these burners in an optimal combustion condition include a method wherein the combustion control of a burner is affected on the basis of a signal obtained by detecting an ionic current which is generated between an electrode provided so as to extend into the tip portion of a flame generated from the burner and a terminal provided on the flame radiating portion of the burner and passing the detected ionic current through a frequency analyzer or other similar circuits, and a method wherein the combustion control of a burner is affected by use of a photosensor which is provided at a position where the flame can be monitored to detect vibration of light, which is then converted into an electric signal and amplified to detect a combustion condition of the flame by means of a frequency analyzer or other similar circuits.
Of the two methods, the combustion condition detecting method that employs the ionic current will first be explained.
The change of the ionic current with respect to the time axis in a certain combustion condition shows an oscillating waveform such as that shown in FIG. 5. In the aforementioned prior art, the oscillating waveform is input to a frequency analyzer to obtain a power spectrum such as that shown in FIG. 6. The power spectrum changes in accordance with a change in the air-fuel ratio, as shown in FIG. 6. Therefore, it has heretofore been a general practice to obtain an integral ratio of the power spectrum in a specific frequency band to that in the entire frequency band, as shown in FIG. 13, obtain a proportional relation of the O.sub.2 content in exhaust gas and the integral ratio (power spectrum ratio) as shown in FIG. 14, and execute combustion control by using the power spectrum ratio as an index in place of the O.sub.2 content (%).
One example of the conventional method that employs a photosensor comprises the steps of: detecting a light power signal from the flame; obtaining a signal representative of the amplitude of the light power from the light power signal; subjecting the thus obtained signal to frequency analysis to obtain a power spectrum; detecting a combustion condition from the power spectrum signal; comparing the detected combustion condition with an optimal combustion condition to obtain a deviation; and controlling a flow controller for combustion air so that the deviation is eliminated. In this method, a semiconductor photosensor, for example, a phototransistor, photodiode, solar cell, etc., is used as a means for detecting a signal representative of the intensity of light emitted from the burner flame.
This method will be explained below more specifically. In a prior art combustion control apparatus shown in FIG. 25, a light power signal a which is representative of the light intensity of a burning flame 3 from a burner 2 provided in a furnace 1 is detected by means of a photosensor 4. The light power signal a is input to a frequency analyzer 7 through a detector 5 and an amplifier 6. The frequency analyzer 7 carries out frequency analysis on the basis of the light power signal a to compute a power spectrum d and outputs it to a light power oscillation controller 8.
The light power oscillation controller 8 computes an integral value J of the power spectrum d in the entire frequency band and an integral value K in a frequency band higher than a specific frequency and divides the integral value K by the integral value J to obtain a power spectrum integral ratio C. The specific frequency is, for example, set as follows: The rate of change of the power spectrum may vary at both sides of a boundary frequency in accordance with the change of the excess air ratio; therefore, such a boundary frequency is, for example, defined as a specific frequency. Further, in expectation that the power spectrum integral ratio C is in approximately proportional relation to the excess air ratio under given conditions, this relation is utilized to control the flow rate of the combustion air 10 by outputting a compensation signal f to a compensator 9 so that the power spectrum integral ratio C is equal to a preset reference value, thereby obtaining a good combustion condition. Temperature controller 17 is provided above compensator 9 and is supplied with a signal from the thermometer 18 and a signal from flow meter 15.
The above-described conventional method wherein the combustion control of a burner is affected on the basis of an ionic current signal detected from the flame has the problems that the combustion condition of the burner is affected by a change in the flame condition caused by the way in which the burner is disposed in the furnace or the flame touching an object to be heated.
Accordingly, it is an object of the invention of this application to provide a control apparatus which is capable of avoiding such adverse effects on the combustion of the burner.
It is another object of the present invention to solve problems experienced when employing the above-described conventional method wherein the combustion control of a burner is affected by sensing the light from the burner flame by means of a photosensor. More specifically, the prior art method suffers from the following problems.
It may be impossible to obtain a high rate of change of the power spectrum integral ratio C, depending upon the arrangements of the furnace and burner.
In the case of an industrial furnace such as that shown in FIG. 26, for example, a heat-insulating wall 11 made from a refractory bricks, etc. is provided on the inner wall portion of the furnace 1 and this heat-insulating wall 11, when heated to high temperature, greatly affects the flame 3 by radiant heat, so that the rate of change of the integral ratio C decreases. For instance, if heavy oil A is burned in the furnace 1 having the heat-insulating wall 11 at 60 l/h and with various excess air ratios, i.e., 1.62, 1.31, 1.17 and 1.05, and the combustion condition is examined by frequency analysis, it is revealed that the highest frequency (about 400 Hz in this example) in the power spectrum d has substantially no change irrespective of the difference in the excess air ratio, as shown in FIG. 27, and consequently the rate of change of the power spectrum integral ratio C obtained is extremely small, as shown in FIG. 28. For this reason, even if such a combustion control apparatus is used for the industrial furnace shown in FIG. 26, it is impossible to satisfactorily control the flow rate of the air 10.